Rhyolite is the most differentiated silicate magma type on Earth and makes up some of the largest explosive eruptions (100-1000's km3), including those that have occurred at Yellowstone National Park. Understanding the origin and evolution of large-volume rhyolitic magmatic systems is of considerable interest because their formation must fundamentally reconstitute and differentiate continental crust, and they are candidates for future "supervolcano" eruptions. The mineral phases in rhyolites often provide a rich opportunity to examine pre-eruptive temperatures, oxidation states, and melt water concentrations, as well as time scales for melt accumulation in the upper crust. However, there is a surprising paucity of phase-equilibrium experiments on natural rhyolite melt compositions, which limits the potential to use these mineral phases to extract the maximum amount of information.
The goal of this proposal is to perform hydrous phase-equilibrium experiments in a cold-seal and piston-cylinder apparatus under controlled oxygen and water fugacity (fO2 and fH2O) conditions on a variety of natural rhyolite liquids over a range of temperature and pressure, which will enable calibration of the plagioclase-liquid hygrometer to rhyolite compositions. The experiments will also be used to determine trace element partition coefficients between mineral and melt, particularly for Sr and Ba. The partitioning behavior is critical to understanding how extremely low-Sr rhyolites form, some of which constitute some of the largest (most voluminous) explosive eruptions on Earth. Finally, these experiments will greatly enhance the calibration of broad thermodynamic models of crystal-melt equilibrium, which when combined with geophysical models can provide a deeper understanding of how large-volume rhyolite magma bodies form and why they sometimes erupt explosively, which is a significant geohazard.
